There are known two-layer reflective screens provided with a reflecting layer for reflecting lights from a projector and a light diffusing layer for diffusing the reflected lights in order to reflect the lights projected from the projector to display images on the screens. For such two-layer reflective screens, used is a reflecting layer exhibiting a substantially constant reflectance for visible light irrespective of the wavelength thereof, such as an aluminum deposition layer or an aluminum paste coating layer, and the light reflected by this reflecting layer is further diffused by the light diffusing layer to enable observation of images with no glares in a comparatively wide viewing angle.
However, such reflective screens also reflect and diffuse light from the surroundings (environmental light) other than light of projected images, when the screens are exposed to such light. Therefore, if projection is performed in a bright environment, light originated in environmental light etc. is reflected and diffused also in intrinsically dark portions of images. As a result, brightness of the dark portions is increased to reduce the contrast of the images, and the images become hard to see. The conventional only way to prevent this phenomenon is darkening the room. However, reflective screens which can reflect high contrast images even in a bright environment have been increasingly desired with the spread of projectors.
Therefore, as reflective screens which can reflect high contrast images even in a bright environment, those formed by successively providing a reflecting layer for selectively reflecting lights of specific wavelengths and a light diffusing layer for diffusing reflected lights on a light absorbing substrate have been proposed (Patent documents 1 and 2). Such reflective screens selectively reflect only light in the wavelength regions of the three primary colors, i.e., blue (B), green (G), and red (R), constituting projector images with the reflecting layer, and absorb light of the other wavelength with the substrate 1, so as to suppress the increase of brightness of dark portions and thereby enable display of high contrast images even in a bright environment.
Used as the reflecting layer of such reflective screens is a multilayer optical film which selectively reflects light of specific wavelengths on the basis of interference of light, and it is designed so as to reflect light in the wavelength regions of blue, green and red entering from the front of the screens as light of projector images.
However, if light obliquely enter into screens using such a reflecting layer, the wavelength region of the reflected light shifts to the shorter wavelength side compared with the reflected light of light incident from the front direction. Due to such a property of the reflecting layer, even an incident light which would be reflected as a white light when it enters from the front direction becomes to be a bluish reflected light when it obliquely enters into the screen, because the wavelength thereof is shifted to the shorter wavelength side.
Such a phenomenon does not pose any problem for light of projector images which usually enters into the screen from the front direction. However, the incidence direction of environmental light is not constant with respect to the screen, and reflected light of environmental light incident with large angles becomes bluish.
Therefore, if an observer is in the front of the screen, the reflected light of the environmental light reaching the observer does not suffer from the shift, and the reflected light of the projector image can be correctly recognized. However, if the observer sees the screen from an oblique direction, there arises a problem that the reflected light of the light from the projector and the bluish reflected light of the environmental light with large incidence angles both reach the eyes, which results in that the whole images are bluely recognized, and thus incorrect image colors are recognized.
Although Patent document 2 refers to reduction of the reflectance for light in the image color wavelength region due to change of the incidence angle of the light of projector images, it does not consider at all the change of observable image colors caused depending on the position of the observer with respect to the screen, and thus reproducibility of image colors is insufficient when the screen is observed from an oblique direction.
Patent document 1: Japanese Patent Unexamined Publication (Kokai) No. 2003-337381 (claim 1)
Patent document 2: Japanese Patent Unexamined Publication No. 2004-138938 (claim 1)